Stiffeners such as roof bows have long been used in the transportation industry as basic structural elements that serve to reinforce the sides and roof of vehicle cargo areas. These structures are commonly installed adjacent to wall members as local reinforcements. Roof bow sections are often formed from high yield strength steel in an effort to increase strength and buckling resistance of the cross sections under various types of loads, including vibration, weather-related (such as snow and ice), and contact loads associated at times with fork lifts and other equipment, as well as the cargo itself, which may impinge upon the section. The strength of the roof bow cross section combined with its ability to interface structurally (often through an adhesive bond layer) with the skin that it reinforces are key to the structural performance of the combined structure.
Stiffeners such as roof bows are important structural members for reinforcing the roof and walls of utility trailers. These members are commonly cold formed by rolling operations to form reinforced utility trailer structures. Such structures permit the designer to achieve maximum strength and stiffness at a minimum weight, while also generally meeting other important installation and in-service requirements. Little has changed during recent decades in regard to the fundamental materials or shapes that are available, other than the introduction of improvements in the metallurgy and surface finish that has made roof bows stronger and more corrosion resistant. Because of ever increasing fuel economy demands and the general need for more economical transport of goods, there is a significant need for improvements that enable roof bows to be lighter, stronger, and more damage resistant than existing designs, while interfacing effectively with adjacent structural elements.
Over the past several years, utility trailer design engineers have been challenged to address ever-higher payload requirements. This has occurred in parallel with weight reduction and other structural efficiency goals that have driven the industry. Ever present safety and structural integrity demands have led the industry to look for more efficient and effective structural members and configurations that are more robust while addressing weight concerns.
Generally speaking, a stiffened utility trailer panel's load-carrying capability is directly related to the type and strength of the stiffener typically installed on one side of the panel. The current approach within the industry to meet higher competitive demands has ranged from resorting to sophisticated alloys, including extruded aluminum and other alloys. It has also included approaches such as using specialty fiber reinforced composite structures, and increasing the depth of conventional stiffener designs as well as the yield strength of the material used in making the stiffeners.
The first, and most common, approach taken by the industry in addressing the higher requirements has been to make conventional stiffeners out of heavier or higher strength materials. These traditional stiffener designs include the C-channel stiffener and Z-shaped stiffener, as well as the hat-shaped stiffener. Heavier gauges such as 0.055 inch min. (17 gauge) to 0.070 inch min. (15 gauge) material are now common. The use of thicker material has not only lead to greater tooling and handling costs, but also has had the effect of simultaneously creating other major problems.
The utility trailer structural panel including any attached stiffeners is a system of parts interacting with each other as they are acted upon by combinations of pressures and in-plane as well as bending loads. Currently, stiffened-panel utility trailer structures found in the floors, side walls, and roofs of utility trailers are typically constructed using steel, aluminum or fiber reinforced composite skins with steel, aluminum, or fiber reinforced composite stiffeners.
In general, metal stiffeners seem to provide greater support as the stiffened panel system sustains bending moments as well as in-plane and pressure forces during service. However, an incompatibility occurs when relatively thick, stiffer sections, i.e., stiffeners made of 0.055 inch min. to 0.070 inch min. material are joined or fastened to thinner, less stiff skin made of 0.027 inch min. to 0.050 inch min. material. The interface area where these two sections are joined is an area of load transfer and thus of relatively higher stress. The reason for this is that the stiffer section resists conforming to the deformation of the less stiff sections as loads are increased. The result is that one part of the system, the skin, tries to slide relative to another part of the system, the stiffener. This may result in early failure of the system, such as by buckling of the stiffener or of the skin. This is due to in-plane loads that result from the constraint that the stiffener imposes on the adjacent skin as the loads are increased. Because of the increased stress at the interface or joining area, manufacturers have been forced to modify parts of the stiffened panel to offset this effect. Because the use of heavier stiffeners increases the shear load through the interface that commonly includes an adhesive layer or fasteners, heavier panels and mounting members have been introduced. One approach to alleviate the problem has been to use stronger adhesive bonds with additional fasteners. This has been implemented in an attempt to alleviate the effects of the high local in-plane compressive stresses that the heavy stiffeners may impose on the panel skin. However, this approach is undesirable because by increasing the number of parts, and increases the complexity and cost of the system.
This approach requires still heavier stiffeners, since the stiffener failure risk is somewhat reduced when it acts as an independent component rather than as part of a fully integrated system. Another drawback to additional fasteners is that it requires substantially more tooling and installation time.
Another approach generally taken by the industry is to make the current generally radial sections including hat-shaped and C-channel stiffeners, deeper and out of thinner, yet higher yield strength material. This offers the advantage of reducing in-plane stress while at the same time increasing bending stiffness due to the deeper configuration. However, this approach has major disadvantages.
First, the thinner materials used in these traditional stiffener configurations make these stiffener sections more susceptible to edge stress concentrations. The conventional C-channel, Z-shaped, and hat-shaped stiffeners have a “blade edge.”This edge is very susceptible to imperfections in the sheet material along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the stiffener and adjacent fasteners or adhesive layers can initiate. Even the most perfect, smooth edge of the conventional stiffener will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads.
Thus, initiation of an edge “bulge” or “crimp” on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or “propagate” easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional stiffener has the effect of enhancing an already established process of failure initiation.
Second, all the above conventional stiffeners, when manufactured out of relatively thin sheet materials, are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the stiffener associated with local compressive or shear stresses. Buckling can precipitate section failure of the stiffener. This in turn causes a stress concentration in the adjacent adhesive bond line or fasteners of the panel skin near the buckled stiffener section, which may cause the stiffened panel system to fail.
Finally, some thinner conventional stiffeners can experience “rolling” when placed under load. Rolling may be caused when the shear stresses within the stiffener result in a net torque about the centroid of the thin walled cross-section thus causing the cross-section to twist, possibly making the stiffener unstable. Another cause of rolling is the curvature of the panel itself that is induced by in-plane or pressure loads that are imposed upon the stiffened panel. Some utility trailer builders have increased the cross-sectional length of the flange furthest from the panel skin of the conventional C-channel stiffener in their attempts to solve the rolling problem, but have been met with only marginal improvement. This is because the increased flange length has had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross-sectional flange length caused difficulty in accessing the fastener areas used in mounting the C-channel to the stiffened panel.